High-purity semiconductor-grade silicon is typically prepared by the so called “Siemens” process where trichlorosilane (TCS) gas is reduced in the presence of hydrogen and deposited onto a heated silicon element. In such a process only about one-third of the silicon fed as TCS is deposited as elemental silicon, with the remainder exiting the reactor as an effluent gas typically comprising greater than 85 mol % unreacted TCS, 5–15 mol % tetrachlorosilane (STC) formed by the dehydrogenation of TCS, up to about 1 mol % of disilane (Si2H6) and chlorodisilanes, and particulate silicon.
In a typical CVD process the effluent gas is then separated by distillation into a low boiling fraction comprising dichlorosilane (DCS) and TCS which is recycled to the CVD reactor and a high-boiling fraction comprising STC, disilane, chlorodisilanes, and particulate silicon. The high-boiling fraction is then further processed in an additional step to separate the bulk of the STC from the other components. This recovered STC can then be hydrogenated to form TCS which is then recycled to the CVD reactor. The remaining components of the high-boiling fraction comprising disilane, chlorodisilanes, and particulate silicon can be further processed to crack the disilanes (hereinafter the term disilane(s) refers to those compounds described by formula HnCl6-nSi2, where n is a value of from 0 to 6) and to separate the particulate silicon therefrom. A typical process for cracking the disilanes is where the disilanes are reacted with hydrogen chloride in the presence of a catalyst, such as palladium on a solid support, to effect conversion to monosilanes, and the particulate silicon is separated therefrom by a process such as spray drying.
Rogers, U.S. Pat. No. 3,933,985, describes a process for converting tetrachlorosilane to trichlorosilane. The process involves passing hydrogen and silicon tetrachloride vapors through a reaction chamber held at a temperature of between 900° C. and 1200° C.
Weigert et al., U.S. Pat. No. 4,217,334, describe an improved process for converting tetrachlorosilane to trichlorosilane. The process involves reacting trichlorosilane with hydrogen at a temperature of 600° C. to 1200° C., with a tetrachlorosilane and hydrogen mixture having a molar composition between 1:1 and 1:50 in equilibrium with trichlorosilane and hydrogen chloride, and suddenly quenching the mixture to below 300° C.
Oda, Japanese Appl. (Kokai) No.11-49508, suggests that hexachlorodisiloxane may be passed over a fixed catalyst along with hydrogen chloride to form chlorosilanes which may be fed into a CVD process for making polycrystalline silicon.
Burgie et al., U.S. Pat. No. 5,118,486, describe the spray drying of a liquid by-product stream containing silanes to separate silicon particles therefrom.
The cited art clearly does not recognize that the effluent gas from a CVD process for preparing polycrystalline silicon can be separated into a fraction containing STC and disilanes and that this fraction can be fed to a reactor for hydrogenation of the STC. Furthermore, the combining in the same reactor of the hydrogenation reaction and cracking of the disilanes provides a benefit to the hydrogenation process of lower required energy input to the reactor and higher yield from the hydrogenation process. This is because while the hydrogenation process is an endothermic equilibrium reaction, the cracking process is exothermic providing heat to the reactor and in addition consumes HCl thereby driving the equilibrium of the hydrogenation process to higher yields of TCS. These reactions are summarized as follows:SiCl4+H2 HSiCl3+HClande.g. Si2Cl6+HCl SiCl4+HSiCl3.
In addition to the above cited advantages, the present invention also reduces the concentrations of the pyrophoric and high-boiling disilanes in the process streams and consequently reduces complications and hazards in the operation and maintenance of process equipment.